1. Field of the Invention
The invention relates to foundations for supporting superstructures and particularly to caissons which are vibrated into the soil for supporting a superstructure on the top end thereof. More particularly, the invention relates to an improved cap adapted to be permanently mounted on the top of the caisson to facilitate the installation of the caisson and to provide an attachment plate for the base of a superstructure to be supported thereon.
2. Description of the Prior Art
There are numerous types, styles and arrangements of foundations which are placed in the soil for supporting a superstructure on the top thereof, whether the superstructure be a building, pole, tower, bridge, etc. These foundations can be metal, concrete or a combination of both. One type of foundation herein referred to as a caisson is a circular or polygon-shaped elongated hollow tubular member with an open bottom, which is vibrated into the soil usually by a sonic or vibratory driver/extractor mechanism.
A sonic or vibratory driver/extractor drives a caisson by being rigidly connected to the caisson, usually by clamps, and by oscillating the caisson into the soil. The vibration "fluidizes" the soil particles in contact with the caisson thereby reducing friction and affecting rapid drive rates. Vibratory driver/extractors incorporate the unique capability of being instantly converted from caisson driver to caisson extractor by merely pulling up on the vibratory crane secondary hoist line.
To install caissons using a sonic or vibratory driver/extractor, the caisson must be raised into a vertical position and the sonic or vibratory driver/extractor attached to the caisson before driving can begin.
Caissons with open tops can be raised into a vertical position by use of lifting holes in the caisson walls near the top end. After the open top caisson is erect, the sonic or vibratory driver/extractor is attached by means of clamps which firmly grip the caisson wall on opposite sides. Upon completion of driving the sonic or vibratory driver/extractor clamps are released and the driving equipment can proceed to the next operation.
After installation of an open top caisson, the superstructure may be attached by various methods including: (1) field welding; (2) anchor bolts embedded in concrete enclosed by the caisson; (3) the lower part of the superstructure may be telescoped inside or outside the caisson top and held in place by bolts and grout.
There are project locations or conditions where use of the above superstructure attachment methods to an open top caisson are not practical or economically feasible. Field welding is prohibited by many specifications. Transportation and placing of concrete and/or grout may be impractical. To minimize superstructure attachment problems, a closed top or capped caisson may be used.
Closed top caissons capped with an integral plate incorporating means of attaching a superstructure may be raised into a vertical position and driven by use of a portable driving head which is bolted to the integral caisson cap before raising and removed after the driving is completed. The portable driving head incorporates lifting holes to aid in raising the caisson and plates for attachment of the sonic or vibratory driver/extractor clamps. Attachment of the portable driving head to the integral caisson cap must be carefully done since the effectiveness of the sonic or vibratory driver/extractor is dependent on a rigid connection. It is difficult and time-consuming to make a rigid bolted connection since the nuts must be tightened sufficiently to minimize gap between the faying surfaces of the portable driving head and the caisson cap. The fact that plates of the sizes involved may have sizeable permissible variations in flatness adds to the difficulties. Use of the vibratory crane to handle the portable driving head is usually not economical, so an additional piece of equipment may be required to assist in installing and removing portable driving heads.
A portable driving head is not required for installing closed top caissons or caissons capped with an integral plate if a pair of diametrically-spaced rectangularly-shaped plates are welded to the caisson top adjacent to its periphery. These plates provide a place for attachment of the sonic or vibratory driver/extractor clamps. Since the clamping plates transmit the vibratory driving forces into the caisson, their alignment and attachment to the caisson top must be done with care.
After installation of a closed top caisson, the superstructure may be attached by various methods including: (1) field welding; (2) bolting into drilled and tapped holes in the caisson top; (3) bolting through holes in the caisson top that match holes or bolts in the mating surface of the superstructures.
In the past, steel plates has been used as the top or cap for the closed top caissons described above. When considerable forces are involved, structural analysis may require not only an excessively thick top plate, but the opening in the center may become so small that access to the inside of the caisson from the top becomes difficult. Liberal access to the inside of the caisson through the top or cap is desirable for: (1) ease of applying and inspection of any internal weld that may be specified between the bottom surface of the caisson top or cap and the inside wall of the caisson tube; (2) removal of dangerous welding gases and fumes which may be hazardous to the worker's health from the partially closed interior of the caisson during application of any interior weld; (3) ease of insertion of bolts through the caisson top in a predetermined bolt pattern which will be used for attaching a superstructure. The bolts normally would be inserted from the caisson interior with the bolt head bearing against the underside of the caisson top; (4) almost unlimited observation of the interior of the driven caisson before a worker puts his hand inside for any reason. The void left inside the top portion of the caisson after it has been driven has been used in the past as a gathering place for wildlife, including snakes.
Accordingly, the need has existed for an improved caisson cap which eliminates the various problems described above which are present in existing caisson top or caisson cap construction. There is no known caisson cap construction of which I am aware which eliminates these problems by providing a cap formed of cast steel having a relatively large central opening in addition to the unique features and advantages described below.